Functioning prototypes show how CO2 (R744) can be successfully used in applications such as bottle coolers, refrigerated containers and mobile air-conditioning without increasing costs or compromising on efficiency as compared to HFCs, according to Stefan Elbel, CEO of Creative Thermal Solutions (CTS), the Urbana, Illinois (U.S.), HVAC&R research company.
Natural refrigerant solutions’ performance, in terms of capacity and efficiency, are on par with HFC baselines, according to Elbel. “I see great opportunities for CO2 in low-temperature heat pump applications and refrigeration in particular,” he said.
Elbel shared the latest natural refrigerants-based technology trends in the Technology Trends session at the ATMOsphere (ATMO) America Summit 2022 on natural refrigerants. The conference took place June 7–8 in Alexandria, Virginia, and was organized by ATMOsphere, publisher of R744.com.
“I want to show these examples to show that technologies for all of these different applications exist,” said Elbel. “Technology to implement natural refrigerants on a wide scale is readily available.”
He highlighted the importance of selecting the right components. “If you do good engineering and select your components wisely, you end up with a relatively conventional [low-cost] solution,” said Elbel.
“CO2 is a good refrigerant for mobile air-conditioning and heat pump applications.”Stefan Elbel, CTS
CO2 bottle cooler performance
Although many bottle coolers today are charged with R134a, and propane (R290) is becoming a strong competitor, it is also possible to use non-flammable CO2 in this application, explained Elbel. This can be done with a relatively simple design that does not include expansion valves, microchannel heat exchangers or variable speed compressors – keeping the costs down.
CTS did a study to compare the performance of various 600L (159gal) beverage coolers that hold up to 600 cans, measuring the pull-down time and energy consumption of R290 and CO2 in comparison to the baseline R134a unit.
R290 outperformed the benchmark R134a system, and CO2 was within 3% of R134a’s energy efficiency at a comparable cost, said Elbel. The pull-down time was improved as compared to R134a. This was achieved through minimal optimization of the CO2 system’s capillary tube and charge.
“This is not to say propane is better than CO2, but rather that there are multiple solutions out there,” said Elbel.
Low-cost CO2 condensing unit a suitable R448A alternative
Elbel also described CTS’s small, low-cost CO2-based condensing unit designed for commercial applications.
Hydrocarbon refrigerant is more difficult to implement in this application due to flammability concerns for larger installations, said Elbel. Although synthetic refrigerant options do exist, these come with larger temperature glides or fall outside of the 150-GWP limit for systems of this size.
The prototype, with a cooling capacity of about 1.6kW (0.45TR), was developed as an example of what could be achieved in a low-cost CO2 condensing unit with the same efficiency and capacity as a comparable R448A unit.
After minimal optimization, CTS was able to achieve the same efficiency and capacity with the CO2 unit as the R448A baseline unit.
Mobile CO2 container outperforms R404A
CTS was asked by the U.S. military to design a piece of specialized equipment – a 6m (20ft)-long multi-temperature refrigerated container for use in a very high ambient temperature (57°C/135°F) environment. The military previously had used R404A in containers and wanted a more sustainable alternative.
The container includes a low-temperature frozen compartment at -20°C (-5°F) that takes up a quarter of the space, with the remaining three quarters for the medium-temperature refrigerated compartment at 3°C (38°F).
“The design challenge was to make the system functional,” said Elbel. The refrigeration system came with severe special constraints and could not exceed 1.8m by 0.6m by 0.6m (6ft by 2ft by 2ft), or weigh more than 550kg (1,200lbs). The design also calls for a cooling capacity of 6kW (1.7TR) and a COP of 1.
To meet the requirements, CTS designed a two-stage transcritical system complete with intercooler and subcritical (booster) compression. It also uses an ejector.
The assembled prototype CO2 container system was able to achieve the required targets, showing an 18% COP improvement as compared to a similar commercially available R404A system.
Potential to replace HFCs with CO2 for air-conditioning
According to Elbel, CO2 is not dead in the automotive industry. “CO2 is a good refrigerant for mobile air-conditioning and heat pump applications,” he said. CTS previously designed a CO2 AC for military applications.
Additionally, CTS was asked to design a system for a high-speed train to replace its R407C air-conditioning system. The company designed a 44kW (12.5TR) conventional dual-circuit system that is very rugged to withstand the harsh operating conditions, Elbel said.
A fully-functional prototype CO2 unit was fabricated for demonstration and evaluation. Results showed a 14% improvement in capacity as compared to the R407C system, plus a 16% COP improvement. “There is great potential for the CO2 unit to replace HFC units,” concluded Elbel.
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